Project Summary The surface expression of nutrient transporters is acutely regulated by the concentration of the nutrient they pump and the metabolic state of the cell. The presence of high nutrient concentrations causes rapid endocytosis and degradation of the corresponding transporter. This negative-feedback system ensures that cytoplasmic nutrient concentration remains in a physiological range. On the other hand, metabolic stress such as glucose or amino acid starvation also triggers rapid turnover of transporters, in this case to safe energy and recover amino acids by protein degradation. This project focuses on a regulatory system that in part determines the turnover rate of yeast transporters: eisosomes and the proton pump Pma1. The majority of proton-driven nutrient transporters localize to plasma membrane structures called eisosomes. Our data suggest that eisosomes function as storage compartments in which transporters are kept in an inactive state. The storage capacity of eisosomes seems to be regulated by the activity of Pma1, a proton pump that uses ATP to maintain the proton gradient across the plasma membrane. High Pma1 activity causes the release of proton-driven transporters from eisosomes, which in turn results in rapid turnover of these proteins. This regulatory system ensures a balance between the proton influx by transporters and proton export by Pma1. Our studies will focus on the mechanism of this eisosome regulation and how the metabolic state of the cell can modulate this system.